Normally you use the nominal value, but it's better to model the resistors for a better guess of what is required rather than using equations to figure it out. You also can use a series resistor alone sometimes and not require the parallel component, or place the resistors multiple places for best application.

Thanks Wolf.
Tweetie is Audax TW025A0.
I think I need to attenuate 3 dB or so. Nominal value is 8 ohms, and using V-Cap's calculator gives 2.3 and 19 ohms for -3 dB. I don't have any test equipment so that'll be a starting point. When you say model what does that entail?
I thought of using a series resistor only; would this affect impedance and lower crossover point?

Modeling involves either tracing, sourcing, or measuring for driver files of spl and impedance, and placing them in various programs to model your xover. Calculators are not indicative of real world results like a simulation can be.

If you place the resistor before the xover circuit in series, then it will minimally affect the circuit as it stands, as well as the xover point. Positioned after- that can tilt your response down, as well as shift the Q of the xover filter (Fc of xover filter remains the same, but the Q shifts) and maybe shift the F3 or F6 of the rolloff to a higher frequency. It depends on the room, boundary locations, drivers used, as well as implementation. Lots more variables than just impedance to look at.